The invention relates to apparatus for indicating variations in acoustic properties at the interface of a first material in contact with a second material. The apparatus has particular application in the detection of liquid in the form of foam, sludge, wet sand, or droplets on a solid surface such as the inside surface of the wall of a tank or pipe.
Known ultrasonic devices have been used for detecting the presence of substances in tanks or pipelines but require firstly that the substance transmits ultrasound and secondly that sufficient substance is present to fill the path between the transmitter and the receiver.
British Patent Specification No. 1,473,840 discloses a different approach in which changes in the acoustic impedance of material loading an ultrasonic transducer are detected.
Detection of variation in acoustic impedance of a liquid caused by the presence of variable concentrations of gas bubbles is described in British Patent Specification No. 1,474,469. The technique employed in this Specification No. 1,474,469 is to measure the diminution in amplitude of an ultrasonic signal pulse passed through the liquid, the path length being effectively multipled by the use of a reflector to secure repeated traverse by the signal pulse across a relatively short length liquid. This technique is not suitable for detection of foam, sludge, wet sand, or droplets on a solid surface because it is not possible to pass ultrasound through such media.